One of the post-machining operations required for the BE8108 gearbox was reducing the outer diameter of the internal gear.  Stock internal gears seem to come with a large amount of outer material, likely because they are intended for stationary process control applications.  For a mobile environment, a 100 tooth gear with a 50 mm pitch diameter is somewhat useless when it has 70 mm outer diameter.

I ended up turning this down on a lathe using a custom 3D printed mandrel.

After getting the stator out of its housing, the next step is to do the post-machining on the rotor.  Here, I didn’t want any of the original bearing housing, and just needed to drill out a hole in the middle big enough to put my sun gear holder through.  I figured I would experiment with some custom fixturing, so broke out Fusion 360 and drew up a set of “soft jaws”… i.e. *really* soft jaws, since they would be 3D printed.

After separating the rotor from the stator, next I needed to remove the stator from the aluminum backplate that holds it into place on the rotor assembly.  It looked like the assembly was at least pressed in, but given that there were no fasteners, it wasn’t clear if it had adhesive, or if perhaps it was a shrink fit.

First, I tried heating up the assembly with a hot air gun, hoping that any adhesive would loosen up and possibly the stator itself would expand or break free.  No dice.

To build the 8108 class gearbox with the geartrain inside, first I need to get the motor apart and separate out the individual pieces that I needed.  Here’s the first in a few posts about the modifications.

The original unmodified 8108 motor looks like:

To begin with, I needed to separate the rotor from the stator.  First, there is a small screwed on plate which needs to be removed – it just has normal Phillips head screws.

As mentioned last time, I’m working on a parallel track to accelerate my quadruped development efforts.  The current plan is to try and use a BE8108 class brushless motor, with a planetary geartrain mounted mostly inside the existing bounds of the motor.

Here’s a rough exploded view of the CAD model:

Key takeaways are:

So, after applying power to the robot for the first time, I coded up some simple scripted maneuvers I was going to use to work up to a gimmick jump video.  Unfortunately, I discovered that one of my assumptions was not well founded, and some more work will be necessary.

Background

I started in on this project intending to create a semi-standard servo motor with integrated gearbox which could be used for all the joints.  The brushless motors I am dealing with are only just barely capable of their task without additional gearing.  Along that development path, I built some prototype integrated gearboxes for a 50 sized brushless motor, and even took some videos of it jumping.

After getting the whole robot assembled, I quickly realized that changing the firmware on each of the 12 servos was going to be a big annoyance.  Doubly so, because the lateral servo programming ports were unreachable with this chassis design without disassembly.  Thus, I bumped up a deferred piece of work to implement a bootloader that would allow for reflashing the primary application over the RS485 communication bus.

Pre-bootloader state

The moteus controller currently uses an STM32F446 controller, which has 512kb of flash memory.  The memory map pre-bootloader looked like:

After getting everything wired up and mechanically assembled, next was zeroing all the encoders and doing the first position control.  I discovered a number of problems that turned up by having a full 12 servos on the bus at the same time, which were resolved pretty easily, as well as many more pain points which I’ll need to address.

But, here is the pretty video (it ends when I push down enough to over-current my lab supply):

Once all the individual legs were assembled, I needed to wire them up into the chassis.  Part of that was building the necessary data harnesses and actually running the power wires from the junction board.

Next up is getting everything talking.

Last time we had a dozen motors, controllers, and brackets assembled.  In this installment we’ll build up the mammal legs.

First is mounting the upper leg and upper pulley on each motor and assembling the lower leg:

Then we stick the upper leg, pulley, and lateral motor together, leaving the lower leg loose: